One of the most difficult problems facing modern aviation is the protection of sensitive instruments and controls from electromagnetic interference (EMI). This problem applies to the pressure transducers that are the subject of the present invention. Such transducers are often used for critical propulsion and flight control functions, and spurious or incorrect information from pressure transducers can result in critical damage to an aircraft.
To avoid signal degradation through EMI, it has become common to provide sensitive transducers with metallic shielding. However, to achieve more complete protection, the wires and connectors carrying signals between a sensor and a control system must also be shielded. This additional shielding adds a significant amount of weight to an aircraft.
One way to reduce or eliminate the EMI problem is to use a passive, all-optical sensor. In such an arrangement, an optical signal is transmitted from a control system to the sensor, the optical signal is affected in some manner (e.g., intensity modulation) at the sensor by the quantity being sensed, and the modulated optical signal is then returned from the sensor to the control system for detection. However, a requirement that a sensor cannot include any electronic components places a severe limit on the types of sensor that can be used, and on the quantities that can be sensed. In addition, all-optical sensors produced to date have been very expensive, and have not been able to match the resolution of electrical sensors.
For sensing pressure, a well-known type of sensor makes use of a piezoelectric (e.g., quartz) crystal mounted such that the crystal deforms in some manner in response to pressure. The crystal is coupled to a suitable drive circuit such that the combination of the drive circuit and crystal forms a crystal-controlled oscillator, i.e., an electrical oscillator whose oscillation frequency follows the "natural" oscillation frequency of the crystal itself. A change in pressure deforms the crystal such that the natural frequency of the crystal changes, causing the oscillator frequency to change. By measuring the frequency of the oscillator, the pressure can be determined.
An optically-powered strain sensor that utilizes a piezoelectric crystal is described in U.S. Pat. No. 4,651,571. In this system, a pulsed optical signal is launched through a fiber-optic cable to a remote sensor. At the sensor, the optical pulses are converted into a DC voltage, as well as into electrical pulses that are used to drive a quartz crystal into oscillation. The oscillation of the crystal generates an AC voltage signal that is fed to a detector that is powered by the DC voltage. The detector amplifies the electronic pulses, and converts them into a corresponding optical pulse train which is transmitted back to the drive circuit along a second fiber-optic cable. At the drive circuit, the returned optical pulses are converted by a photocell into an electronic output signal. The frequency of the output signal indicates the frequency of vibration of the crystal, and therefore of the strain. This output signal is also used as a feedback signal to modify the frequency of vibration of the original drive circuit.
Although the system described in U.S. Pat. No. 4,651,571 achieves the goal of transmitting only optical signals between the control system and sensor, it does so at the expense of a complex design. In addition, the described system, in effect, splits up the oscillator circuit between two sites coupled to one another by a pair of fiber-optic cables, producing a sensing system that is difficult to calibrate in an efficient and reliable manner.